[svlug] summary of nvidia/sil soft raid & Linux soft raid

[Rick Moen]

Quoting Ivan Sergio Borgonovo (mail at webthatworks.it):

> - While it is clear that fakeraid ctls use the cpu to do raid stuff it
> is not clear if *all* the job is done by the cpu.
> http://www.gamepc.com/labs/view_content.asp?id=eoyraid&page=6&cookie%5Ftest=1
> this table *implicitly suggests* that eg. in sil all raid levels
> supported excluding raid 5 are hardware and raid 5 is software, in
> AMCC all the raid levels including raid 5 are hardware.

AMCC (better known as 3Ware) is not fakeraid, but rather real hardware
RAID.

> - looking at the c code in the sata_sil and sata_nv drivers there is
> no reference to raid. That means that even if there was a bit of
> hardware support for raid in those ctls Linux is not able to exploit
> it at the moment.

No.  That just means that support for fakeraid is (in the standard Linux
kernel 2.6.x model) modularised into the dmraid collection of subdrivers,
rather than being included in block device drivers.  The two you mention
(above) are block device drivers.

> - once dmraid will be part of debian install my preference may change
> in favour of the most open driver that at this moment seems to be sil.

I honestly don't know whether Debian's precompiled 2.6 kernels include
the dmraid code.  However, it's not very difficult (post-installation)
to compile a debianised kernel, FYI:  You'll find some guides here:
"Information" on http://linuxmafia.com/kb/Debian/  Of course, it's
probably handy to have that _during_ OS installation.  ;->   I list a
large number of installers for Debian here, listing kernel revisions but
not dmraid status explicitly:  "Installers" on
http://linuxmafia.com/kb/Debian/

(Disclaimer:  As I mention at the bottom of my
http://linuxmafia.com/faq/Hardware/sata.html page, I have very little
personal knowledge of most SATA and SATA/RAID issues, relying mostly on
reading other people's reports and assisstance on the Web and in e-mail.
My SATA Linux page is therefore quite fallible.

> Since in Debian sid amd64, and I suppose other debain versions, dmraid
> is not supported in the installation process, being able to put up a
> system that can be recovered without a rescue disk is a bit painful.

See my "Installers" page:  There is a _lot_ more than just one Debian
installer image -- even for sid AMD64 (though that's admittedly
stretching the point a little).

(A throughly determined person would put in a temporary additional hard
drive without RAID striping, install Debian onto that, fetch or build a
kernel supporting all desired hardware, and then migrate the system onto
the striped set.  However, obviously that _would_ be a pain in the neck.)